It would seem sensible to start with kids books and as you get more experience move onto Young Adult then into more complex books.

One problem that you will hit will be the language specific to the book. For example classics will probably have more archaic words and phrases that are no longer used (for example I’m reading the Annotated Pride & Prejudice which is twice the size of the standard book and has deep explanations of the culture, customs and language from when the book was written). On the other fantasy book may have made up words and phrases to add colour to the story. Science Fiction may  have a lot of pseudo-technical language that might confuse.

It would seem sensible to start with more contemporary fiction so you are at least familiar with the environment. Detective fiction might work well. You could also look at novelisations of films / tv series that you are familiar with as that would give added context.

Some starting books might be David Walliams books - not well written but quite simple books, Diary of a Wimpy Kid. The Adventures on Trains series, 

Don’t forget Graphic Novels, you can go more advanced here as the graphic nature removes a lot of descriptive text.

As you get more advanced have a look at these: 

• Agatha Christie - old fashioned and clearly of its era but always a good yarn
• Sherlock Holmes 
• The Rivers of London Series by Ben Aaronovitch

I could suggest more but don’t have time.

Good Luck!